1. Field of the Invention
The invention relates to a process for reprocessing mixed plastics and to an apparatus for reprocessing mixed plastics.
2. Discussion of the Background
Reprocessing (recycling) mixed plastics, more particularly mixed plastics from household refuse, is becoming more and more important. Through legal guidelines great emphasis is placed on ensuring full recycling of plastics on the home market.
Various processes and apparatus are already known for reprocessing plastics.
German Offenlegungsschrift DE 41 25 164 A1 describes a plant for recycling waste material from plastics material, particularly from agriculture. This contains at least one machine for shredding the waste material which feeds a pre-washing machine for pre-washing the waste articles, a grinding machine for the shredded and prewashed waste and a second washing machine for washing the ground waste. Finally the waste articles are dried in a drying machine and an exsiccation machine.
DE 43 29 270 A1 describes a plant and process for separating shredded plastics of different chemical composition and different density. The plant is characterised in particular through a separating device having the structural design of a geometrical chamber with two oppositely aligned cone parts for producing a laminar fluid flow which makes the desired separation of the plastics possible.
DE 43 06 781 describes a process for reprocessing refuse mixtures which are rich in plastics wherein first the obstructive substances are sorted by hand and then the refuse mixture is shredded and separated by wind sifting into light material containing mainly plastics and heavy material containing mainly metal constituents. The plastics-rich light material is then sorted by means of a wet density sorter according to the density. The density sorting is thereby carried out in several successive stages with different separating fluids, preferably in sorting centrifuges.
The said processes have the drawback that as a result of the wet density sorting or washing processes the reprocessed plastics must first have to undergo expensive drying before it can be supplied for further use.
The moisture of a largely dehydrated ground plastics material clings mainly to its surface. The greater the specific surface of a material so the more moisture clings to it. This produces a corresponding increase in the expense required to dry the ground plastics material. Wet plastics reprocessing is therefore becoming increasingly uneconomic particularly as a result of the trend for thinner and thinner foil articles.
German Offenlegungsschrift DE 39 11 326 A1 describes a process for reprocessing refrigerator housings which consist of moulded plastics parts wherein the refrigerator housings are first shredded in a shredder stage and then the iron parts are removed from the shredded material. The shredded material is introduced into a grinding stage and there shredded to a particle size of less than 10 mm.
The particles of unfoamed plastics are then separated in a wind sifter stage from the particles of foamed plastics. The particles of foamed plastics are introduced into a vortex flow grinding stage and there ground to an end fineness of 0.1 to 1 mm. The ground particles are supplied from the vortex flow grinding stage into a cyclone stage from where the reprocessed foamed plastics is discharged.
This plant which works without wet reprocessing stages is used basically only to separate the foamed plastics from unfoamed plastics of refrigerator housings.
With the dry reprocessing of plastics mixtures, more particularly mixtures from household refuse containing many different sorts, there is the problem of reliably removing from the plastics to be reprocessed obstructive substances, such as paper and the ash which is formed during thermal treatment, in order to allow the effective reuse of the reprocessed material.
The mixed plastics balls from the household refuse collection contain the most varied types of plastics which are used worldwide for packagings. The plastics are connected to paper, ceramics, metals and other foreign matter. These must be removed during the course of reprocessing since they restrict the material usability of the plastics. Thus for example metal remains can lead to premature wear of extrusion machines.